The present invention relates generally to methods and systems of predicting fatigue life in metal alloys, and more particularly to using probabilistic models and high cycle fatigue behavior for predicting very high cycle fatigue life in aluminum and related metals. Even more particularly, the invention relates to predicting fatigue life in cast aluminum alloy objects at very high cycle fatigue levels.
The increased demand for improving fuel efficiency in automotive design includes an emphasis on reducing component mass through the use of lightweight materials in the construction of vehicle component parts, including in the powertrain and related componentry. Cast lightweight non-ferrous alloys in general, and aluminum alloys in particular are increasingly being used in, but are not limited to engine blocks, cylinder heads, pistons, intake manifolds, brackets, housings, wheels, chassis, and suspension systems. In addition to making such components lighter, the use of casting and related scalable processes helps to keep production costs low.
As many of the applications of cast aluminum and other lightweight metal alloys in vehicle components involve very high cycle (generally, more than 108 cycles, and often associated with between 109 and 1011 cycles) loading, the fatigue properties, particularly the very high cycle fatigue (VHCF) properties, of the alloys are critical design criteria for these structural applications. Fatigue properties of cast aluminum components are strongly dependent upon discontinuities (that often initiate fatigue cracks), such as voids and related porosity, or oxide films or the like, that are produced during casting. Moreover, the probability of having a casting discontinuity in a given portion of the casting depends on many factors, including melt quality, alloy composition, casting geometry and solidification conditions. Given these factors, as well as inherent nonhomogeneities of the material, it can be appreciated that the nature of fatigue is probabilistic, where prediction of expected behavior over a range of loads is more meaningful that trying to establish a precise, reproducible fatigue value.
Despite this, there are factors that provide good indicators of fatigue behavior. For example, cracks readily initiate from large discontinuities that are located near or at the free surface of components and are subjected to cyclic loading, and the size of such cracking is important to determining the fatigue life of a component. As a general proposition, the resulting fatigue strength for a given number of cycles to failure, or life for a given load, is inversely proportional to the size of the discontinuities that initiate fatigue cracks.
One particular form of fatigue, known as high cycle fatigue (HCF), is concerned with the repeated application of cyclic stresses for a large number of times. The most commonly-cited value for such large number of times is about ten million (107). The suitability of many structural materials (for example, ferrous-based and non-ferrous based alloys) for use in components and applications where HCF is a concern is often measured by familiar means, such as from the data in well-known S-N curves, examples of which are shown in FIGS. 1 and 2A where the number of completely reversed stress cycles that the material will survive decreases with an increase in stress level. Referring with particularity to FIG. 1, the fatigue strengths and corresponding S-N curves for many materials (for example, ferrous-based alloys) have a tendency to flatten out above a certain number of cycles at a stress known as the endurance limit. In general, the endurance limit is the maximum stress that may be applied to the material through an indefinite number of such completely reversed cycles without failure.
Unfortunately, aluminum-based alloys (also shown in FIG. 1) do not show a clearly-defined endurance limit, instead exhibiting successively lower levels of allowable cyclic stresses, for fatigue lives in the millions to trillions of cycles. Such alloys are considered to be generally not possessive of an endurance limit, or if possessive of one, are such that the endurance limit is not generally discernable or readily quantified. In either event, it is difficult to determine an appropriate design strength (under cyclic loading) and related material properties of cast aluminum alloys beyond either the HCF limit or those associated with very high cycle fatigue (VHCF, typically from about 108 to 1011 or more cycles). Since long-term properties of components made from such alloys are critical to their success and are considered to be important design criteria for these components in structural applications, additional methods of determining strength and related properties for cast aluminum alloys in a manner generally similar to that used to predict the fatigue behavior of ferrous-based alloys are desired.
The well-known Wöhler test (the results of which can be used to produce the aforementioned S-N curve) and staircase fatigue test (the results of which are depicted in FIG. 2B) are commonly used to characterize the fatigue properties of materials for conventional HCF (e.g. 107) life cycles. The statistical analysis of the results of these two fatigue tests is usually based on the assumption that the fatigue strength is normally distributed. As a result, the results generally agree for estimations of median fatigue strength, but show significant differences (up to, for example, a factor of two) in their standard deviation. One of the disadvantages of the staircase fatigue test is that the fatigue strength tested and calculated is restricted to a fixed number of cycles (for example, around 104 cycles for low cycle fatigue (LCF), and 107 cycles for HCF). In comparison with the staircase fatigue test, the S-N curve from the Wöhler test can offer fatigue strengths at different numbers of cycles to fracture. Whether using Wöhler or staircase testing, conventional servo-hydraulic fatigue testing systems operate at nominal frequencies of no more than a hundred or so cycles per second, making it time-wise impractical to generate S-N or related curves for VHCF applications, where 108 through 1011 (or more) cycles are experienced. Accordingly, it would be desirable to be able to estimate strength and related material properties of cast aluminum alloys beyond the HCF limit, including the VHCF range.